1. Field of the Invention
The present invention pertains to a pressure support system for providing a primary flow of gas and a supplemental flow of gas to a patient, and, in particular, to such a system wherein the primary flow of gas is provided by a pressure generator and wherein the supplemental flow of gas to the patient is controlled based on the operating conditions of the pressure generator.
2. Description of the Related Art
It is known to provide a gas flow to a patient using a pressure support system to treat a medical disorder. For example, is it known to use a continuous positive airway pressure (CPAP) device, which is a single-limb pressure support system having a passive exhaust, to supply a constant positive pressure to the patient to treat obstructive sleep apnea (OSA). It is also known to provide a positive pressure that varies with the patient's breathing cycle, patient's effort or based on the condition of the patient, such as whether the patient is experiencing or likely to experience snoring, an apnea, or hypopnea.
Such devices typically include a pressure generator, such as a blower or piston with a pressure regulator or pressure regulating capability, the pressure support device is a variable pressure device, that creates a flow of breathing gas having an elevated pressure. A single conduit, typically a flexible tube, is coupled to the pressure generator to carry the breathing gas to the patient. A patient interface, such as a nasal and/or oral mask, nasal cannula, trachea tube, intubation tube or full face mask, couples the gas flow from the conduit to the patient's airway. The conduit and/or patient interface generally includes a vent for exhausting exhaled gas to atmosphere. The vent is considered a passive exhaust system because there are no selectively actuatable valves that control the flow of exhaust gas associated with the vent.
It is also common to provide a control system to control the flow of pressurized gas to the patient. The control system ranges from the relatively simple, controlling the pressure generator based on inputs from the patient or from a few monitored parameters, to the relatively complex, controlling the pressure generator in a feedback fashion based on monitored conditions of the patient, such as the patient's respiration. In many applications, these devices are used in the home and positioned at the patient's bedside to provide the positive pressure therapy to the patient throughout the night while the patient sleeps wearing the patient interface device.
In many instances, it is also desirable to provide the patient with a supplemental gas, such as oxygen or an oxygen mixture, in addition to the primary flow of gas, which is typically air, provided by the pressure generating system. It is conventional to introduce the supplemental flow of gas into the conduit or at the patient interface device, both of which are downstream of the pressure generator in the pressure generating system.
When introducing a flammable gas or a gas, such as oxygen, which readily promotes combustion, into the pressure support system, safety concerns dictate that steps be taken to minimize the risk of fire. For example, if the pressure generator should fail or be shut off while the supplemental gas, such as oxygen, remains on, there is a chance that the supplemental gas will continue to fill the conduit and/or patient interface and backup into the pressure generator. Typically, pressure generators are designed with this eventuality in mind and are optimized for safety against fire even if oxygen backs up into the device through the conduit, for example, by using electrical circuits that will not produce a spark or reach temperatures exceeding 300.degree. C. during single fault failures. It is also known to provide gas-tight internal airways in the pressure generator so that the supplemental gas can not enter the enclosure in which the pressure generator and its associated electronics are housed. Furthermore, it is also known to provide a check valve in the conduit (breathing circuit) between the pressure generator and the introduction point for the supplemental gas. This valve prevents gas from backing up into the pressure generator if the pressure generator ceases functioning by exhausting the gas in the breathing circuit to atmosphere.
It can be appreciated, however, that creating electrical circuits that will not produce a spark or reach temperatures exceeding 300.degree. C. during single fault failures or creating gas-tight units increase the cost of the entire pressure support device. This is exacerbated by the fact that generally less than half of the pressure support systems on the market are used in conjunction with a supplemental supply of oxygen. Providing a check valve in the breathing circuit between the pressure generator and the introduction point for the supplemental gas, while effective in blocking the backup of the supplemental gas into the pressure generating device, does not prevent a buildup of such gas in the room where the device is located should the above-described scenario occur. This latter point is also true for the first two conventional safety precautions discussed above. The buildup of supplemental gas, such as oxygen, in a room is particularly disadvantageous because of the risk of sparks from other ignition sources, such as electronic equipment, e.g., televisions, telephones, radios, etc, pilot lights and electric or gas heaters.